1. Field of the Invention
The present invention relates generally to a white gold composition for casting, fabricating or soldering jewelry.
More particularly, the present invention relates to a white gold composition consisting essentially of copper, silver, zinc, and manganese.
The present invention also relates generally to a white gold composition and, more particularly, to a white gold composition consisting essentially of copper, silver, zinc, and manganese and further consisting of lesser amounts of tin, cobalt, silicon/copper and boron/copper.
2. General Background and State of the Art
Gold is a yellow metallic element, however, in its pure form it is too soft to be used for general jewelry purposes. In jewelry making, it is standard to mix pure gold with other metals to produce an alloy. Alloys are a mixture of two or more metals. For example, 18 karat gold is comprised of 75% pure gold (24 karat gold) and a mixture of other elements including copper, silver, nickel and/or palladium. Which alloyed metal is dependent on the type and color gold desired (i.e. 18K yellow gold versus 18K white gold). Typically, copper and silver are mixed with pure gold to produce yellow gold alloys, while nickel and palladium are mixed with pure gold to produce white gold alloys.
To date, there are two key elements used to make white gold alloys: nickel and palladium.
There are advantages and disadvantages of using either nickel or palladium.
One advantage that nickel offers is that by “bleaching” out the yellow gold it contributes well to the white color of the gold alloy. The higher the nickel content the whiter the castings. The majority of white gold jewelry is nickel-based. Another advantage of nickel is that it is economical, costing about $1.00 to about $3.00 per ounce.
However, there are at least two negative aspects of nickel white gold: Potential for allergic reactions (i.e. dermatitis) and high casting melting temperatures.
The first disadvantage of nickel-based jewelry products, and the reason it is currently banned in Europe, is due to the increase in the number of allergic reactions to nickel. In particular, once someone becomes sensitized and has an allergic reaction to the nickel, it is permanent condition. Furthermore, the reaction can be quite severe. For example, it has been reported that an estimated 5% of individuals in Europe have allergic reactions to nickel-based metal alloys; while reported cases in the United States are not as high. Hence, the current European solution is to ban nickel as an additive in all metal alloys. However, substituting palladium, another metal used to bleach yellow gold to white gold, continues to be an alternative.
Moreover, nickel is considered to be slightly carcinogenic. Pursuant to Section 301 (b) (4) of the Public Health Service Act, the U.S. Department of Health and Human Services published that nickel and certain nickel compounds are “reasonably anticipated to be human carcinogens based on sufficient evidence of carcinogenicity of nickel and nickel containing compounds in experimental animals . . . and in humans.” (Ninth Report on Carcinogens, 2001, “Nickel and Certain Nickel Compounds,” U.S. Department of Health and Human Services Public Health Service National Toxicology Program; http://ehis.niehs.nih.gov).
In the same report, ingestion, inhalation and dermal contact were noted as the primary routes of potential human exposure to nickel and nickel containing compounds. Also, certain occupations were listed as having significant occupational exposure to nickel, including jewelers.
The second disadvantage of nickel-based metal alloys is that nickel white golds from a casting perspective mandate higher melting temperatures (melting point of nickel is 1455° C., or 2651° F.). Casting jewelry at these very high temperatures can cause sulfur dioxide reactions, or gas porosity. Increase in gas porosity results in poor quality metal alloys. Also, high melting temperatures produce heavy oxide formations, which limit fluidity and reusability when attempting to re-melt.
Lastly in order to increase fluidity and reusability, nickel-based white golds typically use silicon as an additive. Silicon additives may cause a potential problem with silicide hardspots (silicide is an intermetallic compound that forms when the silicon is combined with certain other elements, i.e. nickel, and in the presence of oxygen and pressure). Also silicide hardspots form due to high temperatures as mentioned previously. Thus, various processes of metal casting including the amount of oxygen (air), the melting temperatures and the specific additives determine the size of the silicide hardspots. For example, the greater the oxygen exposure and the longer the cool time will create some considerably large silicide hardspots.
In addition, nickel-based white gold is subject to what is called Stress Corrosion Cracking. Stress corrosion cracking is a metallurgical issue caused by weak grain boundaries that occur in wrought products that are hard and under stress, and can be observed as broken or cracked prongs. The grain boundaries under stress are corroded easily by many chemicals including household variety chemicals.
Thus, the main disadvantages of nickel-based white gold alloys include allergic reactions to nickel; the carcinogenic properties of nickel; high melting and casting temperatures of nickel; and the silicon additive content of nickel-containing jewelry products, which causes silicide hardspots. In short, the disadvantages of using nickel to bleach pure yellow gold to make white gold alloys present some difficult problems.
Palladium-Based White Gold Casting:
The second option to producing white gold is bleaching yellow gold with palladium. Similar to the use of nickel in nickel-based white gold alloys, the higher the palladium the whiter the castings. Unlike nickel-based white gold, higher amounts of palladium produce a white gold that is more gray in color than white.
The disadvantages of using palladium-based white gold are: The high casting melting temperature; the extremely limited reusability of palladium; and the extremely high cost of palladium.
First, the palladium-based white golds mandate high melting temperatures during casting (1900° F.-2100° F.; or 1040° C.-1150° C.). Similar to high casting temperatures necessary for nickel-based white gold castings, high temperatures can cause sulfur dioxide reactions, or gas porosity. In addition, palladium-based alloys tend to solidify very quickly, therefore, perfect gating sprue techniques have to be incorporated in order to reduce gas porosity.
Secondly, palladium has limited reusability due to sulfur pickup by the palladium when using standard investment powders. These additives also increase the hardness of the alloy considerably and not in a positive way. Also, similar to nickel-based white gold alloys, silicon additives cause silicide hardspots.
Lastly, the high cost of palladium, currently up to about $500.00 U.S. dollars per ounce, as compared with approximately $275 U.S. dollars per ounce of gold itself, makes palladium a very expensive option to bleach pure yellow gold to make white gold. Also, the high cost of refining palladium make palladium alloys a very expensive alternative. Hence, when comparing the cost of palladium-based alloys to nickel alloys, palladium-based alloys are much more expensive.
Thus, although palladium is not known to cause any allergic reactions or considered carcinogenic, its high casting melting temperature, limited reusability and high cost do not make it an ideal choice for white gold casting.